A roller bearing supporting a rotational shaft is composed of a plurality of rollers (rolling elements) and a cage retaining a plurality of the rollers. FIG. 4 is a vertical sectional view of an engine using a roller bearing at small and big end portions of a connecting rod. As shown in FIG. 4, the engine has a crankshaft 21 outputting a rotational motion, a piston 22 making a linear reciprocating motion by combustion of a mixture gas, and a connecting rod 24 connecting the crankshaft 21 and the piston 22 to each other to convert the linear reciprocating motion to the rotational motion. The crankshaft 21 rotates on a rotation central axis with the crankshaft 21 taking a rotation balance by a balance weight. The connecting rod 24 has a big-end portion at a lower portion of a straight rod and a small-end portion at an upper portion thereof. The crankshaft 21 and a piston pin 23 coupling the piston 22 and the connecting rod 24 to each other are rotatably supported at the big-end portion of the connecting rod 24 and at the small-end portion thereof respectively via roller bearings 25, 26 mounted on engaging holes respectively.
As the roller bearings 25, 26, a needle roller bearing capable of receiving a high load, although a projected area thereof is small and having a high rigidity is used. The needle roller bearing includes a plurality of needle rollers and a cage retaining a plurality of the needle rollers. The cage has pockets for retaining the needle rollers respectively. A columnar portion positioned between the adjacent pockets retains the interval between the adjacent needle rollers. To decrease a load applied to the needle roller bearings mounted on the connecting rod at the small-end and big-end portions thereof owing to rotations of the needle rollers and rotations thereof on the center of the cage, the needle roller bearings are used in the outside diameter piloting to positively bring the outside-diameter surface of the cage into contact with the inside-diameter surfaces of the engaging holes formed at the small-end and big-end portions of the connecting rod.
On the other hand, in ordinary rolling bearings, the inside thereof is sealed with an inner ring, an outer ring, and a sealing material. The inside of the bearing is provided with rolling elements and a cage. Grease is filled inside the bearing to always lubricate the rolling elements and the cage therewith. On the other hand, because the needle roller bearing is provided with none of the inner ring, the outer ring, and the sealing member, the inside of the bearing is not sealed. Consequently the grease cannot be filled inside the bearing. Therefore when the needle roller bearing rotates, it is necessary to always supply lubricating oil to a sliding-contact portion thereof by a pump and the like.
Because the pump and the like start to operate simultaneously with the start of the rotation of the needle roller bearing, the lubricating oil does not spread to the entire needle roller bearing and thus sufficient lubrication is not accomplished immediately after the needle roller bearing starts to rotate. Therefore friction is generated to a high extent between the cage and the needle roller. Thereby the surface of the cage and that of the needle roller wear, and so do the outside-diameter surface of the cage and the inner-diameter surface of the housing of an actual machine. In the worst case, there is a fear that both are subjected to sear. Therefore to prevent wear and sear which occur immediately after the needle roller bearing starts to rotate, arts of forming a film having a lubricating property on the surface of the cage in advance are proposed.
For example, there is known a method of forming the hard film of diamond-like carbon (hereinafter referred to as DLC) by using a sputtering method on the guide surface of the rolling element of the cage consisting of the steel material having the hard layer formed on the surface thereof by carburizing treatment and forming the film of the soft metal such as silver on the hard film of the DLC (see patent document 1).
The art of directly forming the film of the soft metal on the surface of the cage by a plating method is proposed. For example, the method of forming the silver-plated film having a thickness of 25 to 50 μm on the surface of low carbon steel is known (see patent document 2). The patent document 2 describes that because the silver-plated film decreases the friction between the cage and the needle roller and the friction between the outside-diameter surface of the cage and the housing, the occurrence of sear can be prevented although the bearing is not sufficiently lubricated immediately after the bearing starts to rotate.
There is known the rolling bearing in which the electroless nickel-plated layer is formed on the surface of the cage to impart corrosion resistance thereto (see patent document 3). There is also known the cage for the rolling bearing in which the film consisting of the tin material having the predetermined thickness is formed on the surface of the cage to decrease the rotation torque (see patent document 4).